1. Department of Applied Physics and Physico-Informatics, Keio University, Yokohama 223-8522, Japan
2. School of Materials and Chemical Technology, Tokyo Institute of Technology, Tokyo 152-8552, Japan
3. Keio Institute of Pure and Applied Sciences (KiPAS), Keio University, Yokohama 223-8522, Japan
4. Center for Spintronics Research Network (CSRN), Keio University, Yokohama 223-8522, Japan